Auto-ignition materials in automotive air bag inflators allow the device to safely deploy in the event of a fire. By including an auto-ignition composition the likelihood of a safety hazard resulting from the bursting of an inflator is substantially reduced.
On the other hand, gas generating compositions typically produce relatively larger amounts of gas as compared to auto-ignition compositions. The gas generating composition must not only burn with sustained combustion, it must also liberate the desired amounts of gases within the desired unit of time. Typically, most compositions that are useful as gas generating compositions do not function as auto-ignition compositions because most gas generating compositions do not auto-ignite at temperatures that would make them useful as auto-ignition compositions. Stated another way, most gas generating compositions auto-ignite at temperatures substantially greater than 250 degrees Celsius and are therefore not desirable as auto-ignition compositions within automotive applications, for example. It is therefore an ongoing challenge to optimize the gas generating characteristics with a relatively lower auto-ignition temperature to potentially provide a gas generating composition that in addition to generating gas at useful amounts, also provides auto-ignition function within the same composition, at a temperature substantially lower than the melting point of the gas generating composition.
Accordingly, most inflators or gas generators for vehicle occupant protection systems, for example, typically include an auto-ignition composition juxtaposed next to a gas generating composition. In the event of a fire, the auto-ignition composition ignites to thereby ignite the main gas generating composition for safe management of the gas generating composition. As such, the fire hazard is substantially mitigated.
Other concerns include hygroscopicity of the auto-ignition compositions whereby ignitability and sustained combustion are adversely affected by moisture liberated during aging of the compositions. Yet another concern includes thermal stability and reliable auto-ignition at temperatures below 215 C after testing at 107 C for 400 hours (such as in the USCAR requirements).
An ongoing challenge is to continue simplification of gas generator manufacturing processes thereby resulting in lower overall costs. As such, combining the auto-ignition and gas generating compositions into one composition would simplify the manufacture and assembly of a gas generator, one employed in a vehicle occupant protection system for example.